Recent interest in the external measurement of regional cerebral perfusion with single-photon emission computed tomography (SPECT) has created substantial interest in new brain seeking radiopharmaceuticals. Based on "pH-shift" hypothesis, a new brain imaging agent I-123 HIPDM (N,N,N'-trimethyl-N'-[2-hydroxy-3-methyl-5-[123I]iodobenzyl]-1, 3-propanediamine) has been developed. This agent can be prepared by a simple exchange reaction suitable for routine clinical use. It is currently under phase II clinical trial in this institute and 10 other medical centers. Preliminary results have suggested that this agent is useful for detecting changes in regional cerebral perfusion. We propose to study the metabolism of HIPDM because understanding the in vivo metabolism may better explain the uptake and retention mechanism of this compound in brain and in other tissues. By identifying the major metabolites, the information can be used to provide a model for validating quantitative measurement of regional brain perfusion. In addition, the information on in vivo metabolism of I-123 HIPDM is required by the Food and Drug Administration for any new drug substance before final approval is given for routine clinical use. By examining the chemical structure of HIPDM, it is reasonable to assume that there are several possible in vivo metabolic pathways: demethylation, deamination, N-oxide formation, beta-oxidation, conjugation and deiodination. All of these possible pathways will be explored in vivo, by injecting I-125 HIPDM in rats. Blood, bile, urine and tissue samples will be collected and analyzed using thin layer chromatography (TLC) and high pressure liquid chromatography (HPLC). Authentic metabolites will be synthesized and radiolabeled for comparison and identification. The amount of each identified metabolite will be expressed as the percentage of the total HIPDM and metabolites present in that particular tissue or body fluid sample. In addition, in vitro enzyme assays using isolated rat tissue preparation will be studied to confirm the metabolic pathways.